The genetic pathways and networks that organize the body plan of the mammalian embryo are not well understood. Between implantation and midgestation, the critical events of axis determination, gastrulation, germ layer specification, morphogenesis and early differentiation of organ systems take place, but the majority of genes that regulate these processes have not yet been identified. This is a proposal to continue to use ethylnitrosourea (ENU) mutagenesis to identify genes that control early development in the mouse embryo. Recessive mutations are identified because they cause clear morphological abnormalities in the midgestation embryo. Positional cloning is used to identify the genes responsible for the developmental phenotypes. This approach has identified mutations in 30 genes required for neural and mesodermal patterning, including at least two dozen previously uncharacterized genes. The molecular lesions responsible for 6 mutations have been identified, including mutations in 4 genes that had not been studied previously in the mouse. The work will now focus on the early events of axis specification, patterning and morphogenesis. Recently identified mutations that affect early patterning and morphogenesis will be mapped and analyzed. Several genes that affect these processes will be cloned and characterized molecularly. To increase the efficiency of identification of genes that control early post-implantation development, including axis specification and morphogenesis, transgenic lines will be generated that express fluorescent proteins in defined subsets of cells in the e7.5-e8.5 embryo. These reporter lines will be used to help characterize mutations that have already been identified and to screen for new mutations that affect early patterning and morphogenesis more efficiently. New mutations will be identified based on inappropriate expression of these reporters and define groups of genes that affect specific aspects of early development. Identification of the genes that control development of the mammalian embryo is crucial for the detection, prevention and treatment of human birth defects. The signaling pathways that act in early development also have critical roles in tumor development, so it is likely that these genes identified in this study will define new steps in tumor development and progression.